LESIA - Observatoire de Paris

  • Jeudi 15 mars 2018 à 16h00 (Salle de conférence du bâtiment 17)

    Observations of fast-moving structures in the debris disk of AU Microscopii : 3 years of follow-up with SPHERE

    Anthony Boccaletti (LESIA)

    The instrument SPHERE was installed at the VLT in 2014 and provides a significant gain in terms of contrast with respect to the previous generation of instruments. As a result, we now have access to very high contrast in the close environment of bright stars in particular the young systems in order to search for giant planets and circumstellar disks. During the commissioning in Aug 2014, SPHERE has revealed several structures (several AU in size) in the form of arches or undulations in the midplane of the debris disk around the star AU Microscopii. This disk is seen edge on and the system is conveniently close ( 10 pc) and young as well ( 20 Myr). The comparison of these SPHERE observations with the ones from STIS/HST 4 years before, not only allowed us to re-identify the structures in older data but most importantly led us to conclude that these structures were moving outwards in the disk, some with very large projected speed (4-10 km/s) hence possibly escaping the system. Several assumptions were considered to explain this behaviour, one of the them involves a body in Keplerian motion releasing some dust under the influence of the star’s activity. Since then, the object is regularly observed with SPHERE as part of the GTO and during a monitoring program. After a short introduction on the instrument and its modes as well as the achieved performance, I’ll remind the initial results from 2014 which led to the discovery of these fast-moving structures. Then, I’ll present the recent observations obtained from the last 3 years which unambiguously confirm the motion of the structures. The hypothesis of a parent body emitting an outflow of dust will be discussed in the light of these observations.

  • Vendredi 9 mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Finding new classes of exoplanetary systems with modern direct imaging techniques

    Elodie Choquet (JPL-Caltech)

    Within 2 decades, our classical view of planetary systems and of their formation mechanisms have been revolutionized by the observation of thousands of exoplanets. We now know that most stars host planets, that these planets are surprisingly diverse and often different from our Solar system’s, and that they probably form through a range of complex mechanisms. All these findings were obtained by looking within the first inner AUs only of exoplanetary systems, with indirect observing methods. How do planetary systems look like beyond 5 AU ? How common are planets there, what are their physical properties, how do they interact with the outer disks of dust and planetesimals ? Direct imaging can answer these questions by offering complete views of the outer regions of extrasolar systems. In the visible and near-infrared, the compelling regimes for studying planet atmospheres and dust properties, this observing method faces technical challenges that limit detections to the brightest objects. Here I will present recent works that improve the detection limits of direct imaging instruments, and how they lead to discoveries of a new class of faint objects. I will present my contribution to these developments and analyses, and I will discuss prospects toward detections and characterization of extrasolar systems with JWST.

  • Mardi 6 mars 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Société NanoXplore, FPGA BRAVE, des FPGA durcis européens

    Proposé par Moustapha Dekkali

    NanoXplore est une société pionnière dans la conception de réseaux logiques programmables à grande échelle. Elle est actuellement engagée, au côté du CNES et de l’ESA, dans un programme ambitieux visant à produire une gamme de FPGA durcis européens : les FPGA BRAVE qui offrent à la fois la flexibilité et la performance.

    Le consortium mené par NanoXplore, en partenariat avec ST Microelectronics, a produit un premier FPGA : le NG-MEDIUM, un FPGA reprogrammable durci aux radiations, dont le kit de développement et les outils NanoXmap sont disponibles. Deux autres versions sont à l’étude : le NG-Large et le NG-XLarge.

    La présentation portera sur la gamme de FPGA BRAVE, leurs statuts et les outils associés.

  • Lundi 5 mars 2018 à 14h00 (Salle de réunion du bâtiment 12)

    AO calibration strategy for the ELT : toward a Pseudo-Synthetic Interaction Matrix ?

    Cedric Taïssir Heritier (LAM/ONERA/Obs. Arcetri/ESO)

    To benefit from the full scientific potential of the future Extremely Large Telescope, its instruments will rely on Adaptive Optics (AO) systems. However, the design of the ELT will provide a challenging environment for the AO calibration, as there will be moving elements in the system and no calibration source. To overcome these constraints, a complete rethinking of the AO calibration procedures becomes necessary.

    Some strategies have already been identified and tested on 8-meters facilities such as the VLT-AOF or the LBT-FLAO and the first results seem to lead to a Pseudo-Synthetic approach, merging on-sky measurement and synthetic models. After introducing the context, I will present the current developments of a synthetic model to reproduce the LBT-FLAO Pyramid WFS behavior in the AO simulator OOMAO.

  • Jeudi 1er mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Impact of atomic diffusion on the structure and surface abundances of G and F type stars : stellar parameter determinations and effects of rotation

    Morgan Deal (LESIA)

    Atomic diffusion, including the effect of radiative accelerations on individual elements, leads to variations of the chemical composition inside the stars as well as the surface abundances evolution. Indeed the accumulation in specific layers of the elements, which are the main contributors of the local opacity, modifies the internal stellar structure and surface abundances. Here we show that the variations of the chemical composition induced by atomic diffusion in G and F type stars can lead to an increase of the iron surface abundance and to an increase of the Rosseland mean opacity at the bottom of the surface convective zone. This induces a modification of the size of the surface convective zone, of the radius of the star, of some seismic parameters, and more importantly of [Fe/H]. We also show the effect of the coupling between rotation and atomic diffusion is this kind of stars. These processes need to be taken into account in stellar evolution models as the observations are more and more precise, especially in the context of the future space missions TESS and PLATO.

  • Mercredi 28 février 2018 à 11h00 (Salle de réunion du bâtiment 16)

    Small-scale structures in the upper atmosphere of the Sun

    Krzysztof Barczynski (LESIA)

    Numerous small-scale structures (sizes of the order of megameters) constitute the background for the large-scale structures in the solar atmosphere. Their large number suggests that they play an important role in the energy transport and the magnetic structuring in the solar atmosphere.

    Properties of the small-scale structures in the solar atmosphere will be discussed. Particular attention is given to miniature loops (with a length of approximately 1 Mm) observed for the first time at coronal temperature (> 1 MK), and their relation between the emission of the small-scale structures and the underlying magnetic field. We also make a focus on the structures which are unresolved by modern instruments. We investigate the relation between emission from the different part of the solar atmosphere and underlying magnetic field. This study provides a statistical proxy of the properties of unresolved small-scale structures. We present study based on UV and EUV observation (images, spectra) with a combination of photospheric magnetic field maps.

    We show that miniature loops are a small-scale version of the hot coronal loop. We also find how the correlation and intensity-magnetic field relations (presented in our study as a power-law) change moving up from the upper photosphere to the transition region and discuss possible interpretations of obtained dependencies.

  • Jeudi 15 février 2018 à 16h00 (Salle de conférence du bâtiment 17)

    Active Galacti Nuclei at very high energies - observations and modelisation

    Andreas Zech (LUTh)

    Until now, more than 70 active galactic nuclei, nearly all blazars, have been identified as emitters of gamma rays at very high energies (TeV), by means of Cherenkov telescope networks like the HESS experiment in Namibia. The spectral and temporal information collected from these sources allows to better understand the extreme conditions in the region of emission. The main tool for linking observations to the physics of the sources is the modelisation of spectral distributions incorporating models of radiative transfer. I will present recent observations in this domain and I will discuss their interpretation with leptonic and hadronic models. Projections for CTA will also be discussed.

  • Jeudi 15 février 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Calibration of mixing length parameters with 3D simulation models

    Sonoi Takafumi (LESIA)

    Observation by space missions such as CoRoT and Kepler have provided with a wealth of high-quality data of stellar oscillations. Particularly, rich spectra of solar-like oscillations should allow us to perform precise determination of stellar global parameters such as age, mass and radius, and interior structure. To make the best of such data, we need theoretical stellar models with precise near-surface structure, which has significant influence on solar-like oscillation frequencies. Mixing length parameters of the convection models are a key factor to determine the near-surface structure. However, we have not yet a definitive recipe for giving its value.

    We aim at calibrating values of these parameters across the Heltzsprung-Russell (HR) diagram based on 3D hydrodynamical models, provided by the CO5BOLD code. Although previous calibration with 3D models have limited to the classical mixing length theory (MLT), we analyze also the full spectrum turbulence (FST) models proposed by Canuto & Mazzitelli (1991) and Canuto, Goldman & Mazzitelli (1996). We perform calibration by matching entropy profiles of 1D envelope models with those of the 3D models. For atmosphere of the 1D models, we compare the Eddington grey T-tau relation and the one with the solar calibrated Hopf function based on Vernazza et al. (1981).

    For both the MLT and FST models with a mixing length l=alpha*H_p, calibrated alpha values increase with increasing surface gravity or decreasing effective temperature. For the solar model, the calibrated alpha values for the MLT and FST models with the Eddington T-tau relation are found to be in good agreement with previous works which performed alpha calibration with the Eddington T-tau by matching with the observables of the Sun. It is found that the solar Hopf T-tau relation generally gives photospheric minimum entropy closer to a 3D model than the Eddington T-tau in a wide range of the HR diagram. Since the alpha values substantially vary with effective temperature and surface gravity, 1D computation of stellar evolution should not be performed with the alpha value fixed, but the calibrated alpha values should be implemented into such computation.

  • Mercredi 14 février 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Numerical Simulation of a Global Superflare from Kappa-1 Cet

    Benjamin Lynch (Space Science Laboratory, U. Berkeley, USA)

  • Mercredi 31 janvier 2018 à 15h00 (Salle de conférence du bâtiment 17)

    Modelling of entire prominences with their multiple fine structures : the 3D Whole-prominence fine structure model

    Stan Gunár (Astronomical Institute of the Czech Academy of Sciences, République tchèque)

    We present the 3D whole-prominence fine structure (WPFS) model (Gunár & Mackay 2015) that combines a 3D magnetic field configuration of an entire prominence obtained from non-linear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures within the 3D magnetic model. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region. Thanks to this the 3D WPFS model provides us with a representation of a prominence with complexity that approaches the real prominences.

    This fact was demonstrated by H-alpha visualization of the simulated prominence done by Gunár & Mackay (2015, 2016). To produce the high-resolution synthetic H-alpha images of the WPFS model we use the fast approximate radiative transfer visualization technique developed by Heinzel et al. (2015). This technique allows us for the first time to produce images of simulated prominences in emission on the solar limb and filaments in absorption against the solar disk using a single model. By employing such a visualization we can study connections between the local configuration of the prominence magnetic field and the observable structure of the prominence/filament plasma. In addition, we are able to consistently study the influence of the varying photospheric flux distribution on the prominence magnetic field configuration and its effect on the observable prominence plasma during prominence evolution.

    In addition to the H-alpha line, we have also developed and used a novel technique for synthesis of the emergent radiation at the millimeter/sub-millimeter wavelengths which are employed by the Atacama Large Millimeter/sub-millimeter Array (ALMA) - see Gunár et al. 2016, 2018.

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